The present invention relates in general to radiation measurement, and more particularly to the measurement of low level ionizing radiation.
There exists in the field of health physics a need to monitor low level ionizing radiation in a human environment. The presence of such environmental radiation must not only be detected. Environmental radiation must also be quantified in terms of human tissue equivalent response.
Typically, environmental radiation is measured in terms of microrems per hour (.mu.rem/h). A usual range of measurement is 0-200 .mu.rem/h. The radiation monitored is typically gamma and x-ray radiation from approximately 40 keV to 1.2 Mev.
Environmental radiation measurements allow a health physicist to determine the relative safety of human environments, and to correct problems of, for example, low level radiation contamination.
It is well known in the art that an ion chamber is a good radiation measurement device from the standpoint of tissue equivalent response. However, a typical air-filled ion chamber that is vented to the atmosphere cannot successfully measure environmental radiation in the 0-200 .mu.rem/h range because of its inherently low signal-to-noise ratio. That is, the small direct current signal attributed to environmental radiation impinging on the ion chamber cannot be accurately separated from the direct current background noise of the ion chamber.
To allow the measurement of environmental radiation by use of an ion chamber, the chamber can be pressurized with a suitable gas that provides a higher direct current signal indicative of environmental radiation levels, i.e. provides a high signal-to-noise ratio. A type RSS-111-100 environmental radiation measuring instrument, manufactured and sold by Reuter-Stokes Co. of Cleveland, Ohio, incorporates such a pressurized ion chamber.
While pressurizing an ion chamber with a suitable gas does allow its use as an environmental radiation measurement device, problems do arise. A pressurized ion chamber is a sophisticated device, and hence does not easily lend itself to incorporation in lightweight, portable, radiation measuring instruments. Also, a pressurized ion chamber's accuracy is highly dependent on its maintaining pressure integrity. Even a slight loss of pressure due to a pressure leak can result in inaccurate radiation measurements. Also, the necessary thickness of the ion chamber wall which allows it to function as a pressure vessel, inherently decreases its sensitivity due to the attenuation effect of the relatively thick chamber wall.
It would be desirable to provide a non-ion chamber type instrument for accurately measuring environmental radiation in the 0-200 .mu.rem/h range.